The most problematic part of mobile networks for data transmission is the radio interface. If better-quality speech is to be transmitted or a higher data transmission rate achieved over the radio path, it requires a wider channel-specific bandwidth of the total frequency band which is limitedly available. A compromise has been made in the GSM system between the transmission rate of information and an efficient use of the frequency range. The quality of speech transmission has been improved by advanced speech-coding methods and data transmission sped up by efficient compression methods.
In a digital mobile network, the ability to receive and decode a radio signal is dependent on the carrier-to-interference ratio (C/I) in the location of the receiver. Clearly, too low C/I leads to poor quality or to the loss of the radio link altogether. On the other hand, the quality of radio communication does not become significantly higher with a very high C/I ratio, because the transmission method is designed to adapt to a certain amount of noise so that a signal received above a certain C/I level can be optimally demodulated and decoded. However, too high C/I does not maximize the network capacity especially in normal speech connections. Either the strength C of the carrier should be lowered to reduce the interference caused to other receivers or more interference caused by other receivers should be allowed. This provides means for obtaining a higher capacity from the available radio spectrum. Correspondingly, too high C/I leads to losing capacity. This leads to the known extreme objective that C/I should at each moment be evenly distributed to all receivers of the network.
However, this objective is far from being achieved in the present GSM networks. The following observations are a summary of the current status quo:
The utilisation plan of frequencies is fixed, i.e. one frequency or one frequency hopping diagram is allotted for each transceiver, such as a base station (BTS) or mobile station (MS). This prevents the allocation of a channel, i.e. frequency and time division multiple access (TDMA) time-slot (TS), to a mobile station (MS) so as to achieve an even C/I distribution in the area of the network. Generally speaking, handover (HO) and power control (PC) decisions are not based on C/I, but on other less effective variables, such as field strength (FS) and quality, such as bit error rate (BER). The base station (BTS) can perform some C/I measurements, which can also be called C/N (carrier to noise) measurements, but they are limited and done only in the uplink direction (MS to BTS). For neighbouring cells, only FS measurements are made on the broadcast control channel (BCCH) frequency. Handovers (HO) are made without direct knowledge of the radio conditions on the non-BCCH frequencies. Frequency hopping (FH) enables a statistical interference equalisation, but no active interference control exists at the moment.
A known solution to the problem is disclosed in the Nokia application (No. PCT/FI/99/00876), Dynamically Optimised Channel Allocation (DOCA), which provides an improvement to the present networks. Its most important advantages are:
C/I is determined at the location of each MS and is monitored continuously. This enables the network to detect an insufficient or excessive C/I for each MS and, further, to estimate the total C/I distribution of the downlink transmission path of the network. A local and extensive interference control is made possible.
HOs and PC of the downlink are based on C/I criteria. The network compares the impacts that possible HOs or downlink PC decisions would have on all MSs affected by such a decision. Therefore, HOs and downlink PC decisions are C/I-based. The risk of calls which are cut off due to interference becomes less.
Due to C/I-based HOs, the network can increase C/I for MSs having too low C/I and reduce C/I of MSs having too high C/I, thus distributing C/I to all MSs so as to achieve as even a C/I distribution as possible. An even C/I distribution can be implemented based on either transmission power control or C/I-based handovers HO.
With the exception of BCCH, there is no actual frequency planning in the GSM network. Frequencies are reserved as necessary to allocate channels and for HOs as defined by the C/I examination. Each TS within TRX can be allocated a different frequency, unlike when using fixed TRX-specific frequency allocations. FH is not used, i.e. the frequency used on a given channel does not usually change from one frame to another.
It is known that the quality of speech in mobile communication does not significantly improve after a certain C/I level is reached. Since traffic in the present mobile networks is not only speech but also data transmission, such as circuit-switched AMR (Adaptive Multi-Rate) speech, Internet use by browsers, VoIP (Voice over Internet Protocol) calls and multimedia applications, the quality of service (QoS) has become an important criterion when information being transmitted is to be delivered as correctly as possible. Not only in IP (Internet Protocol) networks, but also in mobile networks numerous different services will be transmitted that can comprise different requirements describing the quality of the service. A higher quality of service in a mobile network requires a correspondingly higher C/I level, in which case the optimisation of the interference level of the mobile network and an even C/I distribution are no longer expedient.
FIG. 1a shows the quality of connections when using different services in the present mobile networks. Both the connections 101 to 105 reserved for voice communication and the connections 106 to 110 reserved for data services receive an equal amount of channels whose quality of connection varies in comparison with what would be the ideal quality of each service. An air interface resource management in which the resources are used for each connection based on the quality of service required by the connection in question is at the moment not available.